Simplify the following expression: $r = \dfrac{1}{2a} + \dfrac{9}{8a}$
Explanation: In order to add expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $2a$ and $8a$ $\lcm(2a, 8a) = 8a$ $ r = \dfrac{4}{4} \cdot \dfrac{1}{2a} + \dfrac{1}{1} \cdot \dfrac{9}{8a} $ $r = \dfrac{4}{8a} + \dfrac{9}{8a}$ $r = \dfrac{4 +9}{8a}$ $r = \dfrac{13}{8a}$